This invention relates to electrographic writing heads for recording information on a dielectric recording medium and in particular to an improved electrographic writing head employing interleaved arrays of writing nibs, small geometry thick film resistors and semiconductor driver circuits fabricated on adjacent glass epoxy substrates separated by a ground plane.
In the prior art, an electrographic writing head ordinarily consists of an array of electrodes, which are either wire wound or deposited on an insulating substrate. The electrodes terminate in writing nibs which are held close to the dielectric surface of a writing medium. The opposite surface of the writing or recording medium is conductive and is coupled with a counter electrode which is held at a predetermined voltage potential relative to the writing nibs.
Low voltage control signal lines may selectively address writing nibs or groups of writing nibs to cause an electrical discharge from the nib to the recording medium. The charge deposited on the recording medium is developed into an image by the application of a liquid or powdered toner which clings to the recording medium by electrostatic attraction between the deposited charge on the recording medium and oppositely charged colored toner particles.
The writing nibs are typically connected together in groups wherein the groups of nibs share control electronics. For example, many writing nibs may be connected together to a single high voltage driver circuit. This creates a multiplexed writing head. The counter electrode behind the recording medium is also segmented with each segment being energized synchronously with its corresponding group of nibs.
Although the multiplexing scheme according to the prior art reduces the number of switching elements, it also adds a considerable amount of complex circuitry. This added complexity for the sake of saving switching elements has several serious disadvantages.
A significant problem in prior art electrographic writing heads concerns the appearance of unwanted bands in written images at the counter electrode boundaries. Because all the writing nibs can not be energized simultaneously, they must also share the time it takes the recording medium to move from one scan line position to the next. This creates the need for relatively high speed and hence high power electronics on the writing heads. The increased power demand of a typical prior art multiplexed writing head raises cost by necessitating expensive power supplies and high power consumption. The increased power demand in prior art electrographic writing heads also reduces reliability.
Yet another problem inherent in prior art multiplexed electrographic writing heads, is that the constant switching of fairly high capacitance nib groups requires expensive high voltage driver circuits with high current sinking capability in order to attain reasonably fast writing speed. Therefore, at maximum plotting speeds, the power consumption of an entire multiplexed nib array can be significant.
Most prior art electrographic writing heads also suffer from a problem known as "flaring". This occurs when the charge deposited on the recording medium does not follow the outline of the nib delivering it, but rather spreads in an uncontrolled manner over the medium. Flaring is caused by excessive discharge from the writing nibs due to the buildup of energy in the capacitance that inherently exits between spatially adjacent writing nibs. Upon discharge of a writing nib, the energy of this stored capacitance may also be discharged, resulting in an arc which may be uncontrolled.
The severity of flaring depends upon the nib-to-nib and nib-to-ground capacitances. If these capacitances can be minimized, the flaring may be reduced, since the stored energy available for causing flares is also reduced.
A further disadvantage inherent in prior art multiplexed writing heads, wherein many nibs are connected to a single high voltage driver, is that plotting speed may be limited due to the need for a minimum write time of 20 to 30 microseconds per writing group. Any less writing time would result in severe image degradation. With an average of 50 nib writing groups, the speed at which one scan line may be drawn is determined by the product of the minimum writing time times the number of writing groups, thus approximately 1000 to 1500 microseconds. This translates into two inches per second at 400 lines per inch resolution or less than 1 inch per second at 1000 lines per inch. It will be appreciated that the prior art is severely limiting for high speed printing applications.
Another disadvantage of a prior art multiplexed writing head is the uneven charge distribution at the fringes of each nib group which may result in image striations or "banding" during the writing and toning process. This is a considerable problem in the prior art and many attempts have been made to minimize uneven charge distribution, but to no avail.
Attempts have been made to control banding by eliminating multiplexing. This is done by providing one high voltage driver circuit for every writing electrode or nib. However, these structures have the disadvantage of taking up prohibitively large amounts of space with so called mother boards, including large and bulky connectors and so called daughter boards which contain large numbers of high voltage drivers as well as pull-up and series resistors. These prior art interconnect schemes are unduly space consuming and are in addition prohibitively expensive and unreliable.
The prior art attempt to eliminate multiplexing through the connection of a high voltage driver with a single writing nib results in significant inter nib capacitance and flaring which occurs upon electrode discharge. In addition, in order to achieve reasonably fast RC writing time constants on the order of 100 microseconds, the value of the pull-up resistors needs to be fairly low. This however, has the disadvantage of high power dissipation and low reliability when several thousand nibs are switching simultaneously.
In the prior art, other attempts have been made to substantially reduce intercoupling capacitance and flaring by using thin film elements in an electrographic writing head. Thin film elements are disadvantageous because they are very expensive to manufacture and require complex processing techniques as compared to thick film elements which may be implemented on printed circuit boards.
Previously, it was thought impractical or impossible to use exclusively thick film elements in an integrated electrographic writing head. The lower limit of writing nib thickness is governed by catastrophic damage to the writing nib end due to disintegration upon application of a high voltage and subsequent discharge. Although it is possible to reduce the energy delivered to the nib, there is a limit as to how far the voltage can be reduced and still obtain a suitable writing discharge. It was further believed however, erroneously, that "[T]he upper limit of nib write end thickness is governed by a thickness that is too large providing too much capacitance and defeating the purposes sought after . . . ". See, for example, U.S. Pat. No. 4,776,450, issued Aug. 23, 1988 at col. 4, lines 24-27.
In view of the foregoing disadvantages of prior art devices, it is apparent that what is needed is an improved electrographic writing head which is able to achieve the seemingly contradictory objectives of maintaining fast RC time constants and high writing speed while minimizing power consumption.
What is also needed is an improved electrographic writing head which has greater reliability while at the same time minimizing inter nib capacitance and consequently reducing flaring and other nonconformities in plotting operations.